In recent years, microwave ovens for cooking and other purposes have been widely used not only in mass-catering and other commercial applications but also in household applications. Microwave ovens are also convenient for cooking in pleasure boats or recreational vehicles. For such uses, therefore, a-c/d-c microwave ovens that can power off either a commercial a-c power source or a battery power source are introduced since these pleasure boats or recreational vehicles usually carry batteries having relatively large capacities.
FIG. 1 is a diagram illustrating the basic construction of a microwave oven that can be operated from either of an a-c or d-c power source, on which this invention is based. In the following, the construction shown in FIG. 1 is termed as a prior-art construction for convenience. In the figure, the output of a transformer 2 for a battery power source DC is connected to the secondary side of an existing (that is, built-in) transformer 1 for an a-c power source AC. On an inverter 3 for converting d-c voltage into a-c voltage is provided to feed power to a magnetron 4 outputting high-frequency energy. Symbol S refers to a power changeover switch. That is, the transformer 1 for the a-c power source AC and the transformer 2 for the battery power source DC are separately provided, and when using the a-c power source AC, high voltage is fed to the magnetron 4 via the built-in transformer 1, and when using the battery power source DC, high voltage is similarly fed to the magnetron 4 via the separately provided transformer 2 by changing over the switch S.
Symbol C refers to a capacitor, and D to a diode.
The prior-art construction described above has the following unwanted problems. That is, the fact that the transformer 1 for the a-c power source AC and the transformer 2 for the battery power source DC are separately provided as high-voltage transformers for generating source voltage for the magnetron 4. This tends to increase the space for transformers and the weight of the entire microwave oven unit, leading to increased size and manufacturing cost.
To overcome these problems, a battery-operated converter using the battery power source DC, is provided, as a substitute for the prior-art construction shown in FIG. 1, to produce a-c voltage having the same voltage and frequency as commercial power source. The output of this converter is connected to an existing microwave oven (having a built-in transformer 1). With this construction, however, there arises the need for high-power converter for commercial power source.
To cope with this, an inverter for converting the battery power source DC to a-c voltage is provided. The a-c voltage of the inverter is applied to a primary winding and another primary winding to which commercial power source is applied are wound on a primary side of a single transformer. A common secondary winding is wound on the secondary side of the same transformer. In this case, however, the output voltage produced across the common secondary winding cannot be kept at the same level for both the commercial a-c power and the a-c voltage from the inverter because the frequency of the a-c voltage applied to the primary winding from the inverter is set at the same frequency as that of the commercial a-c power source, and because leakage characteristics requiring the saturated state of approximately 18,000 gauss of magnetic flux have to be provided when feeding the commercial a-c power, whereas leakage characteristics requiring the unsaturated state of approximately 13,000 gauss of magnetic flux have to be provided when feeding the a-c voltage from the inverter.
Assuming that the frequency is f, the number of turns of the common secondary winding is n, the magnetic flux density is B, and the cross-sectional area of the core on which the secondary winding is wound is S, the output voltage E generated in the common secondary winding can be expressed by Equation (1). EQU E=4.44f.multidot.n.multidot.B.multidot.S (1)
Furthermore, since the voltage applied to the magnetron of a microwave oven is determined by the peak value of the output voltage waveform generated in the common secondary winding, the voltage waveform of the square wave from the inverter, as shown in FIG. 2, has to be higher than that of the sine wave of the commercial power source, as shown in FIG. 3, and the number of turns of the primary winding to which the a-c voltage from the inverter is applied has to be reduced. This inevitably increases magnetic flux B, making this construction impractical.
Next, when feeding power to the magnetron 4 using the battery power source DC, as shown in FIG. 1, the battery power source DC may be overdischarged if the load on the magnetron 4 becomes excessive. This poses some hindrance to the subsequent power source, leading to total failure of the DC battery power source. in extreme cases. This is due to the lack of protective means for the battery power source DC. In such a state, if the battery power source DC is used in common with the power source for driving the engine in large pleasure boats or recreational vehicles, failure of the battery power source DC may make subsequent sailing or driving impossible.
In general, the microwave oven has a safety means for preventing magnetic waves from escaping outside the unit even if the door is opened during peration. The microwave oven of the conventional type has a three-stage switching arrangement consisting of switches SW1 through SW3 to prevent the door from being kept opened to protect users from exposure to microwaves, as shown in FIG. 4. The switch SW3 is a monitor switch that opens when the door is closed. In FIG. 4, the commercial power source AC is fed via the closed switches SW1 and SW2, both of which are closed (at this moment, the switch SW3 remains opened), to a transformer 5 where the voltage thereof is boosted up to a high voltage to feed to the magnetron 4 that produces high-frequency energy. Symbol C refers to a capacitor and D to a diode.
With a microwave oven having two a-c power sources of an a-c/d-c power source, such as an example shown in FIG. 5 having a-c power sources AC1 and AC2, the conventional safety means requires a total of six switches SW1 through SW6, as shown in the figure. This means that as many as six switches have to be turned on and off when the door is opened and closed, making the construction of the door quite complex.
In the microwave ovens having two power sources, including the a-c/d-c dual power source, switches installed on the door must be a small-sized microswitch having a small current capacity due to the construction of the door, which precludes the use of large-capacity switches.
Next, it is desired that in the a-c/d-c microwave oven having the above-mentioned construction, a first primary winding that is driven by the a-c power source, a second primary winding that is driven by the battery power source via the inverter, and a secondary winding connected to the magnetron outputting high-frequency energy are wound on a single transformer. In such a case, in order to generate the same voltage (having the same peak value of the output voltage waveform) on the common secondary winding when the a-c power or battery power is supplied to the transformer, it is desired that magnetic fluxes leak appropriately between the first primary winding and the secondary winding. In the a-c/d-c microwave oven of the conventional type, however, no such magnetic circuits are provided, as mentioned above. It is difficult, therefore, to generate the same voltage on the common secondary winding even when the a-c power or the d-c power is supplied to the transformer.
Since a microwave oven having a magnetron that produces high-frequency energy requires large power, utmost care should be exercised not to cause overdischarging when the oven is driven by the battery power source, as described earlier.
When sensing the discharging state of the battery during the operation of the microwave oven in a pleasure boat or recreational vehicle, the long distance between the battery and the microwave oven may tend to cause voltage drop. This may lead to deteriorated accuracy in sensing the battery voltage.
Next, in the a-c/d-c microwave oven of the conventional type, separate fan motors, turntable motors and other motors are provided for different drive power sources, as shown in FIG. 6. That is, when driving the oven with the a-c power source AC, the fan motor 6a and the turntable motor 7a, both being a-c motors provided on the side of the a-c power source AC, are operated, and when driving the oven with the battery power source DC, the fan motor 6b and the turntable motor 7b, both being d-c motors provided on the side of the battery power source DC, are operated.
The microwave oven has safety measures consisting of switches SW1 through SW5 that interlock with the door to prevent magnetic waves from escaping outside the unit even when the door is opened during operation. SW3 is a monitor switch that opens when the door is closed.
When driving with a-c power, the voltage of the a-c power source AC is fed to the transformer 5 via the closed switches SW1 and SW2 (at this time SW3 remains opened) and boosted to a high voltage in the transformer 5 to feed to the magnetron 4 producing high-frequency energy.
When driving the oven with the d-c power, d-c voltage is applied via the closed switches SW4 and SW5 to the inverter 3, where the d-c voltage is converted to an a-c voltage to feed to the transformer 5.
With the arrangement shown in FIG. 6 above, provision of separate fan motors 6a and 6b and turntable motors 7a and 7b for a-c and d-c power sources would be contrary to the miniaturization requirement for such cardboard or shipboard equipment.
When the output of the inverter 3 is a commercial frequency of 50 Hz or 60 Hz, the fan motor 6a, the turntable motor 7a and other motors provided on the side of the a-c power source can be driven by a square-wave voltage induced in the primary winding of the transformer 5 when the oven is driven by the d-c power. If the inverter 3 is operated with a frequency higher than commercial frequency, 200 Hz, for example, commercial-frequency motors provided on the side of the a-c power source cannot be driven by such a high frequency.
Next, in the microwave oven of the conventional type, output changeover is performed in such a manner that when output is changed to the HIGH side, the timer switch TS provided on the power line, as shown in FIG. 7, is operated in the continuously ON state, and when output is changed to the LOW side, the timer switch TS is operated in the ON state for 5 seconds and then in the OFF state for the subsequent 5 seconds.
The microwave oven has safety measures consisting of three-stage switches SW1 through SW3 that interlock with the door to prevent magnetic wave from escaping outside the unit even when the door is opened during operation, as shown in FIG. 7. SW3 is a monitor switch that opens when the door is closed.
In FIG. 7, the voltage of the a-c power source AC is fed to the transformer 5 via the closed switches SW1 and SW2 (at this time SW3 remains opened) and boosted to a high voltage in the transformer 5 to feed to the mangetron 4 producing high-frequency energy.
With the output changeover arrangement in the conventional microwave oven using the timer switch TS, a special-purpose switch has to be provided. In the microwave oven having two a-c power sources or an a-c/d-c power source, two special-purpose switches have to be provided.